If I pass away, how does my spouse gain access to my keychain data?

That will be easier, after the upgrade to macOS 12 Monterey, coming this fall: macOS Monterey Preview - Apple

See the paragraph about "AppleID":

"Apple ID. Account Recovery Contacts make resetting your password and maintaining access to your account easier than ever. And a new Digital Legacy program lets you designate people as Legacy Contacts so they can access your account in the event of your death."

As others have already posted, iOS/MacOS “version next” is due to address this very real concern.

However for now, here’s Apple’s “official public guidance” on the matter:

https://support.apple.com/en-us/HT208510

If iCloud Keychain is set up in Mac Set up iCloud Keychain - Apple Support

For iPhone Set up iCloud Keychain and iCloud Passwords - Apple Support

Reveal your admin password of Mac so that she can view passwords from keychain access Manage passwords using keychains on Mac - Apple Support

If she don't remember the password the admin password can be reset from recovery mode the most important is iCloud Keychain security code , if you don't knew it overwrite as it is required after setting up admin password , as pop up starts appearing for webmail account / Apple ID .

( a ) Open system preferences > iCloud > the box for keychain is checked , click on options , click on change security code .

( b ) Click on security code > enter new code in the fields and click on next , you have to enter the new code for the second time and click on next .

In the next window the country code is written , enter your trusted mobile number …….and click on done .

Again enter your Apple ID password to update your account settings and click on ok , hence the code is changed , so write down this code in a safe place .

Also website passwords can be viewed from safari preferences Change Passwords preferences in Safari on Mac

If you are having Mac and iPhone share passwords Share passwords using AirDrop with Safari on Mac - Apple Support

You must reveal your Apple ID address , if account is set up with secondary authentication in which three answers for security questions , date of birth , rescue mail address and its password , credit card details reveal them to your wife .

As to access the account Apple ID and password these credentials are required , if passcode is used in iPhone one must remember it .

She can then view passwords in keychain access View the information stored in a keychain on Mac - Apple Support

For web accounts its mail address / secondary mail address and their passwords are to be revealed .

The data like passwords are end to end encrypted and apple can't view the credentials , it's the account holder who remember them , write down in a diary .

Make a list of your passwords and accounts and keep it in a place where she will get access to them. This could also be digital if you have a way to encrypt the file with a master password.

This is good you are thinking of this. The concept of a digital legacy is becoming increasingly important.

Note that the How to request access to a deceased family member's Apple accounts - Apple Support

document is about getting access to Apple's accounts, as does the Monterey preview seem to suggest. This issue goes beyond those. If you are in charge of paying the electricity bill, does your wife know how to pay an electricity bill due 2 days after your passing without having to march down to the electricity company office with legal documentation?

Read the document several of us have provided as a link. Apple has to meet a balance between giving people fair access to equipment and accounts while still respecting the privacy of even deceased people. You need to consider if something Apple might keep private even if you are deceased is something you don't really care about if your wife sees it and she may need. Also note that Apple will require legal documentation such as a court order. A list of major accounts on a piece of paper or in digital format will just make things a lot easier.